Polysulfone resin has been widely used to prepare separation membranes. However, since it is hydrophobic, its membranes tend to precipitate impurities, resulting in dramatically decreased water flux. It has been suggested to convert the polysulfone resin into a sulfonated polysulfone resin (C. Friedrich et al., "Asymmetric Reverse Osmosis and Ultrafiltration Membranes Prepared from Sulfonated Polysulfone," Desalination, 36 (1981), 39-62), or into aminated polysulfone (Japanese Provisional Application (A) No. 62-42704). The sulfonated or aminated polysulfone membranes have increased hydrophilicity and a wider range of uses.
In conventional processes for converting aminated polysulfone resin to the desired membrane, a solution of the polymer is formed containing a water-soluble additive, e.g., water-soluble polymers, organic compounds, or inorganic salts. Examples of water-soluble polymer additives include polyethylene glycol, polypropylene glycol, and polyvinylpyrrolidone. Examples of water-soluble organic compounds include ethylene glycol, glycerol, and propylene glycol. Examples of water-soluble inorganic salts include lithium nitrate and lithium chloride.
The solution having the water-soluble additives denoted above are conventionally cast onto a woven or non-woven reinforced cloth and the solvent thereafter evaporated. The cast reinforced cloth is immersed in a gelation medium, such as ice water, to form the membrane product.
In Japanese Provisional Application (A) No. 62-42704, water-soluble lithium nitrate is used in a casting solution of aminated polysulfone. The solution must be evaporated at a temperature of over 50.degree. C, and, to prepare a suitable membrane, the processing times used are 40, 50, 60, and 70 minutes. For this reason, the process is unsuitable for use with a conventional wet casting machine and high industrial production rate cannot be achieved.
In sum, such conventional processes and the membrane thereby obtained have significant disadvantages. They cannot effectively be employed for mass production when using a wet casting machine. High production rate cannot be realized because of the long period for evaporation of solvent.
Moreover there is the desire to produce an aminated separation membrane of high permeability to water, as well as a membrane having improved ability to separate and recover materials having cationic functional groups, such as dyestuffs, amino acids, and cationic electro-deposition paint.